Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for sensor applications, such as in chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the SAW sensor. Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure.
An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave (SAW/SH-SAW) and amplitude plate mode (APM/SH-APM) devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers (IDTs) placed on a piezoelectric material. Surface acoustic wave devices may have a delay line, a filter or a resonator configuration. Bulk acoustic wave devices are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of mask. Based on the foregoing, it can be appreciated that acoustic wave devices, such as a surface acoustic wave resonator (SAW-R), surface acoustic wave filter (SAW-filter), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), can be utilized in various sensing measurement applications.
One promising application for micro-sensors involves oil filter and oil quality monitoring. Diesel engines are particularly hard on oil because of oxidation from acidic combustion. As the oil wears, it oxidizes and undergoes a slow build-up of total acids number (TAN). A pH sensor is capable of direct measurement of TAN and an indirect measurement of total base number (TBN), providing an early warning of oil degradation due to oxidation and excess of water. The acids and water build-up is also related to the viscosity of the oil.
Low temperature start-ability, fuel economy, thinning or thickening effects at high and/or low temperatures, along with lubricity and oil film thickness in running automotive engines are all dependent upon viscosity. Frequency changes in viscosity have been utilized in conventional oil detection systems. The frequency changes caused by small changes in viscosity of highly viscous liquids, however, are very small.
Because of the highly viscous loading, the signal from a sensor oscillator is very “noisy” and the accuracy of such measurement systems is very poor. Moreover, such oscillators may cease oscillation due to the loss of the inductive properties of the resonator.
There is a need to provide a sensor system which can be utilized to monitor, in a sensitive manner, the etching effects of etchants, such as acids contained in oils. There is also a need to provide a sensor system which can monitor corrosion or degradation of engines or other devices caused by exposure to such etchants. It is believed that acoustic wave sensors may well be suited for such monitoring as indicated by the embodiments described herein.